1. Field of Invention
This invention relates to an electrical isolation circuit and more particularly to an opto-coupler electrical isolation circuit.
2. Discussion of The Prior Art
An opto-coupler has found wide acceptance in many applications as a means of isolating electrical signals from each other. For example, peripheral equipment associated with computer systems are often subjected to large transient currents which flow into the ground loop or ground line and as a result cause the ground line to rise in voltage. Servo motors are often required to drive output devices such as tape decks and necessitate output voltages in the 150 V and upward range. The transient or surge currents often cause the ground line to rise by more than 5 or 6 volts which in turn causes a feedback from the output to input devices and back to the main frame of the computer, thus triggering erroneous circuit operation. An opto-coupler is an ideal structure for positively eliminating this feedback in an inexpensive and reliable manner.
Existing opto-couplers are extremely expensive and require complex amplifying circuits on one hand, or in the alternative operate at inadequate switching speeds and signal levels thus making them undesirable and impractical for many applications.
One prior art opto-coupler employs a radiation source, such as a light emitting diode, and a silicon diode detector. The diode detector is constituted by a monolithic structure comprising a substrate, an N epitaxial layer, and a surface diffused P-region for forming the PN junction. With the PN junction reversed biased, a minority carrier current is generated by the movement of electrons from the P-diffused region into the epitaxial layer and the movement of holes from the N epitaxial layer into the P-diffused region. The miniority carrier current primarily occurs by the generation of electron hole pairs in the N epitaxial region upon the incidence of radiation. This type of detector suffers from major drawbacks.
It is recognized that for integrated circuit application it is desirable to provide conventional active devices on the same chip or substrate with the detector. Accordingly, it is most advantageous that the detector device be capable of being fabricated by standard integrated processing techniques. Most advantageously these techniques dictate that the epitaxial thickness be in the order of five to ten microns. However, if the prior art detector is fabricated in a thin epitaxial layer much of the incident radiation passes through the two active regions constituting the PN junction, that is, the upper surface planer P-diffused region and the N epitaxial region without being completely absorbed. Consequently, the surface diode detector is incapable or is limited in the magnitude of current which is capable of generating from the incident radiation. For example, with a sixteen milliamp current being applied to a gallium arsenide LED radiation source and the conventional silicon diode detector located in the N epitaxial region, approximately 5 to 8 microamperes of current is generated, and this low level of generated current in turn gives rise to two attendant problems.
Firstly, the switching response time of the detector is inversely proportional to the generated current. By way of example, for 10 microamperes and a 10 micro-micro farad capacitor associated with the detector (a capacitor in parallel with a current source is the diode detector equivalent), it would take approximately 5 micro seconds to generate a 5 volt signal across the detector. This response time is unacceptable from many applications and accordingly would require additional integrating circuits to increase the response time, or would require high beta transistors or Darlington configurations with the obvious attendant disadvantages.
Secondly, the low magnitude of current generated by the detector would require a great number of sophisticated and costly amplifying stages in order to be capable of generating a sixteen milliampere output current from a 10 micro ampere detector current. As many as 15 or 20 transistors sometimes are required for this amplification function is some known prior art units.
An ostensible solution to this problem caused by incomplete absorption of the incident radiation within the active regions is to increase the epitaxial thickness. An epitaxial thickness of thirty microns has been suggested in order to insure substantially complete absorption by the active regions employed to generate the miniority carrier current in the device detector and thus increase the magnitude of the generated currents. An apparent additional solution is to increase the N epitaxial resistivity from 1 ohm cm. to 30 ohm cm. However, these alternatives create severe processing problems for optimized integrated circuit implementation.
The processing time to form junction isolation in a thick epitaxial region is extremely long and would cause extensive surface damage as well as being difficult to control. Other forms of isolation processes would also be extremely expensive, impractical, or difficult to achieve with extremely large epitaxial thicknesses, or diffused layers.